Step by step guide of the CMake build system of this library

In this guide, we will open up the CMakeLists.txt file and examine it part by part

Lets begin with ./CMakeLists.txt, the main file

cmake_minimum_required(VERSION 3.1.0)

set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR arm)
set(CMAKE_C_COMPILER "arm-linux-gnueabi-gcc")
set(CMAKE_CC_COMPILER "arm-linux-gnueabi-gcc")
set(CMAKE_CXX_COMPILER "arm-linux-gnueabi-g++")
set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED true)

Defines the system we are building on and the cmake version we are using Compilers need to be specified so that CMake doesn’t get confused in the Docker container and break, or cry out errors

set(LIBNAME "ev3-cpp-template-wrapper-lib")
project(${LIBNAME} VERSION 2.0.0)
message(STATUS ${CMAKE_CXX_FLAGS})

Defines name of library, the version of the library (it will be changed to 2.5.0 later) and outputs the cmake flags during the build. Even though

message(STATUS ${CMAKE_CXX_FLAGS})

is no longer needed, it is still useful for debugging purposes

set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
add_compile_options(-Os)

Make CMake export compile commands into a file that vscode can process and use as intellisense. This is just for vscode intellisense add_compile_options(-Os) tells the compiler to build the library while optimizing for size (minimising binary size) so that the robot can load the executable faster

add_library(
    ${LIBNAME}
    STATIC

    lib/ev3dev.cpp
    lib/blockable/ev3wrapblockable.hpp
    lib/button/ev3wrapbutton.cpp
    lib/button/ev3wrapbutton.hpp
    ...
    lib/ev3dev.h
    lib/ev3wrap.hpp
)

Actually defines the library .a compilation goal. The files listed here contain the source code of the library.

target_include_directories(
    ${LIBNAME}
    PUBLIC
    ${PROJECT_SOURCE_DIR}/lib/
    ${PROJECT_SOURCE_DIR}/lib/blockable/
    ${PROJECT_SOURCE_DIR}/lib/button/
    ${PROJECT_SOURCE_DIR}/lib/color/
    ...
)

Tell CMake where to find the specified files from the previous part (add_library)

set_target_properties(${LIBNAME}
                    PROPERTIES
                    RUNTIME_OUTPUT_DIRECTORY ${PROJECT_BINARY_DIR}/bin/
                    OUTPUT_NAME "ev3dev-cpp-template-wrapper"
)

Set the output directory and the output file name

set("WRAPPER_EXAMPLES_DIR" ${PROJECT_SOURCE_DIR}/examples)
set("WRAPPER_SCRIPTS_DIR" ${PROJECT_SOURCE_DIR}/scripts)
set("WRAPPER_BIN_DIR" ${PROJECT_BINARY_DIR})
set(WRAPPER_BIN_FINISHED_DIR ${WRAPPER_BIN_DIR}/finished)
set(WRAPPER_BIN_EXAMPLES_DIR ${WRAPPER_BIN_FINISHED_DIR}/examples)
set(EVERYTHING_BUILT_TARGET "EVERYTHING_BUILT_TARGET")
set(LIB_BUILT_TARGET "LIB_BUILT_TARGET")

Define a list of variables that will be used later WRAPPER_EXAMPLES_DIR : directory containing the example source code WRAPPER_SCRIPTS_DIR : directory containing the bash scripts used for compiling WRAPPER_BIN_DIR : directory containing the output binary (build folder) WRAPPER_BIN_FINISHED_DIR : directory where the final finished binaries will be copied to WRAPPER_BIN_EXAMPLES_DIR : directory where the finished examples will be copied to EVERYTHING_BUILT_TARGET, LIB_BUILT_TARGET : The name of a target. This will be used later

add_custom_target(
    ${EVERYTHING_BUILT_TARGET} 
    ALL
)
add_custom_target(
    ${LIB_BUILT_TARGET}
    ALL
)

Add the 2 targets that keep track of:

1. If everything, including the executable using the library has been built
2. If everything within the library has been built

These targets are important for when we want to measure the binary size of the library once the build is finished.

add_dependencies(${EVERYTHING_BUILT_TARGET} ${LIBNAME})
add_dependencies(${LIB_BUILT_TARGET} ${EVERYTHING_BUILT_TARGET})

Make EVERYTHING_BUILT_TARGET depend on LIBNAME and LIB_BUILT_TARGET depend on EVERYTHING_BUILT_TARGET This is just making sure the targets make sense

add_custom_command(
    TARGET ${EVERYTHING_BUILT_TARGET} POST_BUILD
    # setting up the directories
    COMMAND sudo mkdir -p ${WRAPPER_BIN_FINISHED_DIR}
    COMMAND sudo mkdir -p ${WRAPPER_BIN_EXAMPLES_DIR}
    # commands for the library itself
    COMMAND sudo cp ${WRAPPER_BIN_DIR}/libev3dev-cpp-template-wrapper.a ${WRAPPER_BIN_FINISHED_DIR}/libev3dev-cpp-template-wrapper.a
    COMMAND sudo bash ${WRAPPER_SCRIPTS_DIR}/post_build.sh "The-Ev3Wrap-library" 500 ${WRAPPER_BIN_FINISHED_DIR}/libev3dev-cpp-template-wrapper.a 50
)

Make it so that once everything is built, the binaries are copied to the finished folder, and post_build.sh is run to check its size.

add_subdirectory(examples)

Add another CMake file stored within the examples folder. That other CMakeLists.txt file manages the compilation of the examples.

We will continue looking at CMakeLists.txt in the examples folder here